The present invention relates to semiconductor devices, and more particularly to a method for forming a polysilicon (i.e., poly Si) layer over a high-dielectric constant, i.e., high-k, dielectric material, wherein an interface exists between the poly Si and the high-k dielectric material which is uniform and has high thermal stability. Semiconductor structures such as field effect transistors (FETs) and capacitors comprising at least a poly Si layer formed over a high-k dielectric material, wherein an interface exists between the poly Si and the high-k dielectric material which is uniform and has high thermal stability, are also provided herein.
Dielectric materials in high-density circuits appear as capacitors in dynamic random access memory (DRAM) applications, gate dielectrics in transistors and as decoupling capacitors. The dielectric materials in these structures are typically silicon dioxide (SiO2), silicon nitride (Si3N4) or any combination thereof These dielectric materials typically have a relative dielectric constant of 8.0 or below.
As today""s generation of circuits become smaller and smaller, the dielectric materials employed therein must be made thinner to satisfy circuit requirements. The use of ultra-thin, conventional relatively low-dielectric constant materials in today""s circuits is undesirable since such materials lead to leaky circuits. Thus, it would be beneficial if the dielectric constant of the dielectric material used in such circuits could be increased.
A variety of high-dielectric constant, i.e., high-k, materials such as binary metal oxides including aluminum oxide (Al2O3), zirconium oxide (ZrO2), hafnium oxide (HfO2), lanthanum oxide (La2O3), titanium oxide (TiO2), as well as their silicates and aluminates; and perovskite-type oxides including a titanate system material such as barium titanate, strontium titanate, barium strontium titanate (BST), lead titanate, lead zirconate titanate, lead lanthanum zirconate titanate, barium lanthanum titanate, barium zirconium titanate; a niobate or tantalate system material such as lead magnesium niobate, lithium niobate, lithium tantalate, potassium niobate, strontium aluminum tantalate and potassium tantalum niobate; a tungsten-bronze system material such as barium strontium niobate, lead barium niobate, barium titanium niobate; and Bi-layered perovskite system material such as strontium bismuth tantalate, bismuth titanate are known in the art.
Fabrication of integrated circuits using a high-k dielectric material to replace SiO2 has encountered a major problem with poly Si/high-k interface stability. For example, during the activation annealing at 1000xc2x0 C., 5 seconds interface instabilities are formed in the structure which greatly degrade the electrical properties of the device.
This degradation is clearly seen in prior art FIG. 1. This figure shows an interface of poly Si/ZrO2 after being annealed at 1000xc2x0 C., 5 seconds. Note that the film is not uniform and strong reactions are present at the poly Si/ZrO2 interface. The arrow shows that part of the film is etched away. In this prior art figure, the poly Si was deposited over the high-k dielectric material using a conventional low-pressure chemical vapor deposition (LPCVD) reactor. The wafer containing deposited high-k dielectric material was heated in the reactor chamber at about 620xc2x0 C. and then an undiluted Si-containing precursor gas, e.g., SiH4, was introduced in the reactor and a poly Si film was deposited on top of the ZrO2.
In view of the instability problem that exists in prior art poly Si/high-k dielectric film stacks, there is a continued need for developing a new and improved method for forming a poly Si layer over a high-k dielectric material in which the resulting interface between the poly Si layer and the high-k dielectric is uniform and thus has a higher stability as compared with prior art poly Si/high-k dielectric film stacks.
One object of the present invention is to provide a method of forming a poly Si layer over a high-k dielectric material in which the interface between the poly Si and the high-k dielectric material is uniform.
Another object of the present invention is to provide a method of forming a poly Si layer over a high-k dielectric material in which the interface between the poly Si and the high-k dielectric material is thermally stable, i.e., no reactions between the poly Si and the high-k dielectric material are observed during high-temperature activation annealing.
A further object of the present invention is to provide a method of forming a poly Si layer over a high-k dielectric material using a technique that is compatible with existing complementary metal oxide semiconductor (CMOS) and/or back-end-of-the line (BEOL) processing steps.
A yet further object of the present invention is to provide a method of forming a poly Si layer over a high-k dielectric material which avoids the use of an undiluted Si-containing precursor gas.
A still further object of the present invention is to provide a method of forming a poly Si layer over a high-k dielectric material in which less hydrogen gets incorporated into the resultant poly Si/high-k dielectric film stack.
These and other objects and advantages are achieved in the present invention by employing a method wherein the poly Si layer is deposited using a Si-containing precursor gas that is diluted with an inert gas. The term xe2x80x9cSi-containing precursor gasxe2x80x9d as used herein denotes a gas which includes silicon and hydrogen. The inventors have unexpectedly found that improved poly Si/high-k dielectric interface stability can be achieved if the poly Si deposition is carried out in a Si-containing precursor gas that is diluted with an inert gas such that the hydrogen content that is being incorporated into the resultant film is less than that of an undiluted Si-containing precursor gas.
Specifically, the method of the present invention comprises the steps of:
(a) heating a semiconductor structure containing an exposed surface of high-k dielectric material to a temperature of from about 350xc2x0 to about 750xc2x0 C. inside a deposition reactor chamber, said high-k dielectric material having a dielectric constant, k, of greater than 8; and
(b) depositing a layer of polysilicon on said exposed surface of said high-k dielectric material, wherein said depositing is performed in said reactor chamber using a Si-containing precursor gas which is diluted with an inert gas whereby said layer of polysilicon has a lower hydrogen content than that obtained from an undiluted Si-containing precursor gas.
In one embodiment of the present invention, an interfacial oxide, oxynitride, and/or nitride layer is formed on the structure prior to deposition of the high-k dielectric material.
Another aspect of the present invention relates to semiconductor structures which are fabricated utilizing at least the method of the present invention. The inventive semiconductor structure comprises:
a semiconductor substrate having an upper surface;
a layer of high-k dielectric material formed on said upper surface, said high-k dielectric material having a dielectric constant, k, of greater than 8; and
a layer of polysilicon formed on said layer of high-k dielectric material, wherein an interface exists between said high-k dielectric material and said polysilicon that is uniform.
The term xe2x80x9cuniformxe2x80x9d as used herein denotes an interface region that is continuous, i.e., the interface has no breaks therein. Moreover, the interface region of the present invention is substantially planar and it follows the contour of the underlying high-k dielectric material. Because the interface region is uniform, the high-k dielectric material has a thickness that is substantially constant.
Examples of semiconductor structures that are contemplated in the present invention include, but are not limited to: FETs, and capacitors. In some embodiments of the present invention, especially when the semiconductor substrate is a Si-containing semiconductor substrate, an interfacial oxide, oxynitride, and/or nitride layer is formed on the upper surface of the semiconductor substrate and then the high-k dielectric is formed on said interfacial layer.